This invention is concerned with molten metal coating of metal sheet material and in particular relates to novel apparatus which provide faster and more efficient production, better product control and improved product.
Although there have been many highly productive improvements in metal coating operations over the last 25 years, molten metal coating itself, especially control of coating weight, has remained essentially the same. Prior to this invention, molten metal coating operations, have relied on mechanical contact with strip at the exit side of a coating bath. This has been a slow cumbersome process, making coating weight control one of the biggest drawbacks and bottlenecks, especially in continuous strip practice.
Since hot dip zinc and zinc alloy coating, herein termed galvanizing, is the commonest form of molten metal coating operations, the invention will be described in this environment.
The invention makes a radical departure from prior art practice by providing a coating control apparatus which accurately determines coating weight in continuous strip galvanizing operations. The coating control apparatus of the invention leaves the strip free from the marks and damage occasioned by coating rolls, and the like, eliminates changing and cleaning of such mechanical contact devices, and provides numerous unexpected advantages such as increased line speeds, better operational control, a choice of manual or automatic coating weight control, smoother finish, more uniform coatings, and better corrosion protection with less consumption of coating metal.
There are pronounced contrasts between the teachings of the invention and past theories on wiping coating advanced for hot-dip tinplating. For example, the U.S. patents to Steele U.S. Pat. No. 850,548, Sebell U.S. Pat. No. 2,370,495, Sherman U.S. Pat. No. 2,390,007, and the British Patent Specification No. 588,281 disclose use of a liquid or equate use of a liquid and compressed gas in tinplating. There are similar contrasts between the teachings of the invention and the wiping action of a high velocity stream of steam passing between a coated surface and internal surfaces of a throat to blow excess metal from the surfaces of a material as disclosed in the U.S. patents to Underwood U.S. Pat. Nos. 2,080,518, 2,095,537 and U.S. Pat. Re. No. 19,758. Such theories have no application in the galvanizing industry and in fact none of these prior art theoretical disclosures is known to have found practical application in hot-dip metal coating of any kind. In practice, coating rolls, despite their many shortcomings and difficulties, remain in use throughout the strip steel galvanizing industry.
The present invention overcomes these problems by controlling coating with what is herein termed a gaseous barrier. Coating control by gaseous barrier leaves the strip free from the marks and damage occasioned by coating rolls, eliminates changing of rolls and other mechanical problems and provides numerous unexpected advantages such as increased line speeds, better operational control, smoother finish, and so forth, which will be discussed below.